This invention relates to panels having double wall construction with an evacuated space enclosed between the walls, useful as thermal insulating panels for refrigerators, picnic coolers, freezers, refrigerated trucks and the like.
Foamed plastic panels are currently widely employed as insulating panels in refrigerators, refrigerated trucks, picnic coolers and the like. However, the thermal conductivity of said foamed plastic panels is approximately three to six times as great as that of a vacuum panel. Therefore, in a given time, up to six times as much heat is lost through a foamed plastic panel as is lost through a vacuum insulating panel of like dimensions. Moreover, the insulating properties of foamed plastic panels often deteriorate with time.
Therefore, it would be highly desirable to provide an economical vacuum insulating panel. In U.S. Pat. No. 3,179,549 to Strong et al. such a vacuum insulating panel is disclosed. Said panel, however, is of all metal construction, and due, to the high thermal conductivity of metal, significant heat is lost through the edges of the panel. U.S. Pat. No. 3,993,811 to Walles discloses a plastic vacuum panel, the walls and edges of which are rendered substantially impermeable to gases by depositing thereon a layer of metal and a layer of a barrier polymer. While this plastic panel does not lose significant amounts of heat through its edges, pinholes or other imperfections in the metal layer may permit gases to permeate the walls of the panel at a rate such that the useful life of the panel is less than desired for some applications.
In view of the deficiencies of known vacuum thermal insulating panels, it would be highly desirable to provide a vacuum panel having improved thermal insulating properties and a prolonged useful life.